The invention relates to an error correction encoding/decoding apparatus which is used in a digital automobile telephone or the like.
Hitherto, in a system using a radio transmission of a digital automobile telephone or the like as a transmission line a strong error correction encoding is performed so that an information data transmission quality of a predetermined level or higher can be held even in case of an inferior transmission path quality (high line bit error ratio). There is a convolutional code as one of such strong error correction codes. As a decoding method of the convolutional code, a Viterbi decoding for performing a maximum likelihood decoding by using a trellis diagram is known. The Viterbi decoding has been disclosed in "Points of Error Correction Encoding Technique", Japan Industrial Technology Center Co., Ltd., published on Mar. 20, 1986, pages 45 to 48.
FIG. 1 shows a construction of a conventional error correction encoding/decoding apparatus. FIG. 1A shows a block diagram on the encoding side. FIG. 1B shows a block diagram on the decoding side. In FIG. 1, an example using a low-bit-rate coded voice signal is shown as an example of an information signal for error correction encoding.
In FIG. 1A, reference numeral 21 denotes a voice signal which is converted into a low-bit-rate coded voice signal by a voice encoding circuit 22. Reference numeral 23 denotes a convolutional error correction encoding circuit for error correction encoding all of or a part of the converted low-bit-rate coded voice signal. Reference numeral 24 denotes a low-bit-rate coded voice signal which has been error correction encoded. The signal 24 is transmitted as a transmission signal by a radio transmission.
In FIG. 1B, reference numeral 25 denotes a low-bit-rate coded voice signal which has been error correction encoded. The signal 25 is error correction decoded by a convolutional error correction decoding circuit 26. Reference numeral 27 denotes a voice decoding circuit to decode the error correction decoded low-bit-rate coded voice signal to a voice signal 28.
FIG. 2 shows a construction of the convolutional error correction encoding circuit 23. As an example of the encoding, a encoding rate (ratio between the number of bits of the information signal and the number of bits of the transmission signal) R=1/2 and a constraint length (the number of information bits regarding the generation of the transmission signal) K=3.
An information signal series 29 is supplied to shift registers 30 and 31 in accordance with the order of X.sub.0, X.sub.1, X.sub.2, . . . . The exclusive ORs of the input value of the information signal series 29 and output values of the shift registers 30 and 31 are calculated by exclusive OR circuits 32, 33, and 34, respectively, so that transmission signal series (convolutionally coded signals)35 and 36 are obtained. The transmitting order of the signals which are transmitted by the radio transmission is set to Y.sub.0, Y.sub.1, Y.sub.2, Y.sub.3, . . . .
FIG. 3 shown a state transition diagram called a trellis diagram which is used when the convolutional error correction decoding method (Viterbi decoding) is executed. States S.sub.0 to S.sub.3 indicate values of the shift registers 30 and 31 in the coding. S.sub.0 indicates (R.sub.0, R.sub.1)=(0, 0); S.sub.1 (R.sub.0, R.sub.1)=(1, 0); S.sub.2 (R.sub.0, R.sub.1)=(0, 1); and S.sub.3 (R.sub.0, R.sub.1)=(1, 1).
The trellis diagram is a diagram in which a branch to the transition destination state in the case where 0 or 1 has been supplied as an information signal in each of the states S.sub.0 to S.sub.3 is shown for continuous information signal bit series. For example, in case of S.sub.2, when 0 is supplied as an information signal, the next state is set to S.sub.0, and when 1 is supplied, the next state is set to S.sub.1, so that the state is branched from S.sub.2 to S.sub.0 and S.sub.1. A branch line from each of the states to the next state is called a branch. Continuous branches corresponding to the information signal series X.sub.0, X.sub.1, X.sub.2, . . . are called paths. All of the information signal series (each of X.sub.0, X.sub.1, X.sub.2, . . . can have 0 or 1) can be expressed by either one of the paths of the trellis diagram.
The Viterbi decoding is a method whereby reception signal series Y.sub.0 ', Y.sub.1 ', Y.sub.2 ', Y.sub.3 ', . . . and transmission signal series Y.sub.0, Y.sub.1, Y.sub.2, Y.sub.3, . . . corresponding to the respective paths (namely, the respective information signal series) are compared for all of the paths, the maximum likelihood series as a transmission signal series is discriminated, the information signal series corresponding to the maximum likelihood transmission signal series is determined to be the information signal series X.sub.0, X.sub.1, X.sub.2, . . . which has been convolution encoded on the transmission side, and it is decoded into the information signal series.
As mentined above, even in the conventional error correction encoding/decoding apparatus, even if a line error has occurred in the transmission signal series Y.sub.0, Y.sub.1, Y.sub.2, Y.sub.3, . . . and it has been changed to the reception signal series Y.sub.0 ', Y.sub.1 ', Y.sub.2 ', Y.sub.3 ', . . . on the decoding side, so long as such a line error lies within an error correcting capability of the convolutional code, the information signal can be correctly reproduced on the decoding side by the Viterbi decoding.
In the above conventional error correction encoding/decoding apparatus, however, there is a problem such that when the line bit error ratio is high and exceeds the correcting capability of the convolutional code, the correct information signal cannot be reproduced by the error correction decoding and a bit error remains.